1. Field of the Invention
This invention relates to a method for the destruction of perchlorate in a ferric chloride and hydrochloric acid aqueous regenerant solution used for the regeneration of anion exchange resins when the latter of which have been used for ground water remediation. More particularly, it relates to a destruction technique for perchlorate that does not change the chemical properties of the ferric chloride and hydrochloric acid regenerant solution accomplished by using controlled temperature/pressure and chemical reagents. The regenerated elution mixture may then be recycled or reused for the regeneration of anion-exchange resins loaded with perchlorate.
2. Background of the Art
Perchlorate (ClO4xe2x88x92) has been widely used as a rocket propellant and in munitions in the United States and abroad, and improper disposal of perchlorate-containing materials has resulted in a significant new threat to groundwater and drinking water supplies. Its distribution is widespread, as documented by Damian, Environmental Protection, Jun. 24, 1999, and Urbansky, Bioremediation Journal 2, 81 (1998). Because ClO4xe2x88x92 ions are nonvolatile, highly soluble, and kinetically inert in dilute aqueous solution, they cannot be effectively removed from water by conventional carbon filtration or sedimentation methodologies or by chemical reduction with reducing agents commonly used for environmental remediation such as elemental iron or dithionite. On the other hand, treatment by ion exchange, using highly selective anion exchange resins in particular, has proven to be an effective technology for removing ClO4xe2x88x92, especially at low concentrations. The ion-exchange technique has been viewed by many as the preferred method for the removal of perchlorates, and various resins have been studied for removal of perchlorates. For example, Kawaski et al., Radiochimica Act 63, 53 (1993) reported adsorption on DOWEX(trademark) 1-X8. Gu et al., in Perchlorate in the Environment, pp 165-176 (2000) found that one bed volume of a bifunctional resin (Purolite(trademark) D-3696) was able to treat greater than 100,000 bed volumes of groundwater before a significant breakthrough of ClO4xe2x88x92 occurred (with an initial ClO4xe2x88x92 concentration ofxcx9c50 xcexcg/L orxcx9c0.5 xcexcM/L).
Once the resin reaches its loading capacity, the spent resin is regenerated for reuse. In other words, the entire treatment process involves the use of selective anion exchange resins to remove prechlorate in water and the regeneration of spent resin bed. However, because ClO4xe2x88x92 anions are selectively and strongly sorbed as a result of their low hydration energy and large size, regeneration of spent resin becomes a particularly challenging and costly task. Indeed, a major operating cost for the ion exchange process is the disposal of spent regenerant solution. For example, Calgon Carbon Corporation""s ISEP process produces approximately 1% of the volume of the treated groundwater as regenerant wastes (NaCl brine), which must be disposed of. The combined brine and its disposal costs are well over $350 per acre foot of water treated on the basis of a 1% rejection rate.
Our previous U.S. patent application (Ser. No. 09/491,242, now U.S. Pat. No. 6,448,299), incorporated herein by reference, disclosed a novel methodology to regenerate spent perchlorate-loaded resins using a combination of ferric chloride and hydrochoric acid solutions (Gu et al., Environ. Sci. Technol. 35, 3363-3368, 2001; Gu et al., Remediation, Marh 2002). Tetrachloroferrate (FeCl4xe2x88x92) anions, formed in a solution of ferric chloride and hydrochloric acid (e.g., 1 M FeCl3 and 4 M HCl), were found to effectively regenerate spent resins loaded with perchlorate. A mass-balance analysis indicated that nearly 100% recovery of ion-exchange sites was achieved by washing with as little as xcx9c2 bed volumes of the regenerant solution in a column flow-through experiment. There was no significant deterioration of the resin""s performance with respect to ClO4 removal after repeated loading and regeneration cycles. Our new regeneration method produces regenerant waste of less than 0.005% volume of the treated groundwater and, therefore, offers a more cost-effective means to regenerate ClO4 loaded resins with improved regeneration efficiency, recovery, and waste minimization than conventional NaCl brine regeneration techniques.
However, despite the fact that our new regeneration technique offers greatly improved efficiency and waste minimization of the volume of the regenerant, the production of hazardous perchlorate-containing waste regenerant solutions, and their disposal, remain to be issues of great concern. The need exists, therefore, for a method to completely destroy perchlorate in the ferric chloride-hydrochloric acid regenerant solutions and, more importantly, to allow the reuse of the regenerant solution. The methodology must not change the properties of the regenerant solution so that the solution can be reused in many regeneration cycles. Ideally, the perchlorate-destraction process is efficient and cost-effective, while not being subject to difficult-to-maintain operating conditions, nor generating any secondary wastes. The method should also be suitable for both the treatment of large aquifers or bodies of standing water as well as for localized facilities to provide potable water on site.
Li and Coppola, EPA STAR Report, 2000, disclosed a hydrothermal/thermal technique to decompose perchlorate in a 7% NaCl brine solution with added reducing agents and promoters. They reported that only partial destruction of perchlorate could be obtained by the additions of ferric salts to sodium chloride brine and essentially no perchlorate decomposition was obtained with or without other promoting agents. Their Integrated Thermal Treatment Process included a reverse osmosis step to concentrate the brine and the nitrate and sulfate rejection systems, but the process is believed to be ineffective for the destruction of perchlorate in the ferric chloride and hydrochloric acid solution used in the process of U.S. Pat. No. 6,448,299.
It is an object of this invention to provide a method for the decomposition of perchlorate eluted from an anion exchange column using a ferric chloride and hydrochloric acid regenerant solution. It is a further objective of this invention to provide a process for recovering and recycling a regenerant solution used to regenerate a spent anion exchange column which has been used to remove perchlorate from water. It is yet a further objective of this invention to provide a system for routinely regenerating an anion exchange column used for perchlorate removal and to recycle the regenerant solution in a cost efficient manner.
These and other objects of this invention may be achieved by a process which removes perchlorate and regenerant from an anion exchange column, introduces a small quantity of non-toxic organic solvent (0 to 5% vol/vol preferably  less than 2% vol/vol) ferrous ions, 0.01 to 20% wt/vol) and/or a catalyst to the mixture, provides sufficient energy to drive a redox reaction to completion, and purges and holds the regenerant for recycle. More specifically, the eluted perchlorate together with regenerant FeCl3/HCl solution are diverted to a reactor vessel, (either a batch reactor or a continuous flow-through system), mixed with an organic solvent or a catalyst and maintained at a relatively low temperature (from ambient up to xcx9c300xc2x0 C.) and pressure (generally  less than 500 psi) that is sufficient to decompose perchlorate. Ferrous chloride and organic alcohols are preferred chemical reagents in view of availability, cost, and effectiveness. Subsequently, the solution is cooled and ready for reuse when a resin column needs regeneration.